Recently, the biological activities of glycoconjugates such as glycoproteins, glycolipids etc. in animal cells have been revealed and the importance of sugar chains in glycoconjugates comes to be recognized. Usually, sialic acid is existed at the nonreducing terminals of sugar chains in glycoconjugates, and sialic acid is considered to have particularly many functions where the physiological functions and biological activities of sugar chains are regarded as important. So, the synthesis of sialoside is very important to examine the (biological or physiological) effect of sialosides, such as disaccharides, oligosaccharides, sugar chains in glycoconjugates, etc.
For linking of sialic acid via an .alpha.-2,6-linkage to the 6-position of monosaccharides such as galactose, mannose, N-acetylgalactosamine and N-acetylglucosamine or to the 6-position of a galactose residue present at the nonreducing terminal of a free sugar chain derived from glycoconjugates such oligosaccharides, glycoproteins or glycolipids, chemical synthesis method and enzymatic synthesis method are known. The chemical synthesis method is flexible and nearly limitless as compared with the enzymatic synthesis method, but the procedure is complicated and side reactions also occur, so it is extremely difficult to efficiently synthesize a product having sialic acid linked selectively to the 6-position of its galactose.
On the other hand, the enzymatic synthesis method is extremely simple as compared with the chemical synthesis method and no side reactions occur, so it is possible to synthesize the desired product in high yield. Enzymes used therein are obtained from internal organs such as the submaxillary gland, liver etc. in animals such as rats, pigs, humans etc. (Poulson et al., J. Biol. Chem. 252, 2356-2362 (1977), Weinstein et al., J. Biol. Chem. 257, 13835-13844 (1982), Miyagi et al., Eur. J. Biochem. 126, 253-261 (1982)). However, it is disadvantageous that these enzyme are expensive because they cannot be obtained in large amounts due to their difficult purification.
A certain animal-derived sialyltransferase has already been cloned, but sialyltransferase has still not been supplied inexpensively in large amounts. Therefore, the synthesis of sialic acid-containing oligosaccharides or glycoconjugates is extremely difficult under these circumstances.